This invention relates to fluorinated phyllosilicate materials and their production. It is particularly concerned with incorporating fluoride ions into a phyllosilicate structure to alter the physical and chemical characteristics of the material.
Essentially any phyllosilicate, whether of natural or synthetic origin, may serve as a starting material. However, the natural materials are generally of greater interest from an economic standpoint. The silicate minerals of interest include vermiculite, beidellite, nontronite, volchonskoite, saponite, stevensite, sauconite, pimelite, bentonite, montmorillonite, hecorite, the smectites, attapulgite, sepiolite, phlogopite and biopyrobole.
Sheet silicates of the mica type are built of two units, viz., a tetrahedral sheet and an octahedral sheet. The former consists of tetrahedra of Si-O linked together to form a hexagonal network such that the bases thereof are coplanar and the apices thereof point in the same direction. This configuration yields a Si:O ratio of 2:5. In constrast, the octahedral sheet is generated through the impingement of two tetrahedral sheets pointing toward each other and crosslinked by the sharing of oxygens by Mg (or Al, Fe) in octahedral coordination. The two octahedral corners not falling in the plane of apical oxygens are occupied by hydroxyl or fluoride ions. It is possible that a composite sheet formed in this manner will be electrically neutral, in which case Van der Waals-type forces bond in to the sheets immediately above and below. More commonly, however, an excess negative charge exists due either to ion substitutions or unoccupied sites (vacancies) or a combination of both. Differences in properties arise from both the degree of charge deficiency as well as the location of the excess charge. Charge balance is restored through the uptake or foreign cations in interlayer positions in 12-fold coordination due to hexagonal rings of oxygens located in the sheets above and below.
In order to create a product from vermiculite, it is usually necessary to delaminate the particles. This involves separating the crystals at the interlayer to form high aspect ratio platelets. These may be suspended as a gel and subsequently deposited in any desired form, such as a sheet, or otherwise processed.
The silicate layer units in these minerals have a thickness of about 10 Angstrom units, with the main elemental constituents being Mg, Al, Si, and O.sub.2. These silicate layers are separated by an interlayer composed of water molecules associated with cations, such as Mg.sup.++, Na.sup.+, K.sup.+ and H.sup.+.
The three layer micas in general, and natural vermiculite in particular, have been extensively studied because of their potential for thermal resistance and electrical insulation. The interest has been heightened considerably with the recent flight from asbestos products.
Many of the phyllosilicates, however, tend to be quite hygroscopic. Various solutions to this problem have been proposed. For example, it is known that adsorbed water molecules and hydroxyl ions may be removed by thermal treatment. This can be very effective, particularly if carried out under reduced pressure. However, there is usually a strong tendency to rehydrate after the material cools and is exposed to ambient conditions.
It has been noted that sites on phyllosilicate structures may be occupied by either hydroxyl or fluoride ions. It is also known to synthesize micas, generally structured fluormicas, and other phyllosilicate-type structures containing fluoride ions. For example, U.S. Pat. No. 4,297,139 (Beall et al.) discloses water-swellable, synthetic materials, most desirably glass-ceramic materials, which may be used to prepare gels, and, in turn, fibers, papers, films and the like. The structures of the synthetic micas and other phyllosilicates are similar to those of the naturally-occurring materials. The Beall et al. patent further discloses cation exchange in the synthetic materials of the patent.